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Please comment.

I'm thinking of investigating linear operation at reduced power, possibly up to 25W (~16v PS).

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While you could...

*Expect lower power maybe 10-15W,? side effect of linear operation.

*Expect poor IMD .?

* it may require negative feedback to insure stability and improve linearity.

*Individual bias would help but would be hard to implement. Layout and

circuit does not permit without extensive changes.

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If you want decent power 20-24V is a must.??

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Look at the K5OOR/EBY design for clues and changes.

There are several other designs for linear operation.

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Allison
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See article:

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Its a two part in files.

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Allison
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I can confidently say it is not as easy as you may imagine.? In the design notes Hans lists a number of things left out of the design because it is not going to be linear and also a number of other issues spring to mind (e.g. Dissipation levels, TX/RX diode bias may be a problem used in dynamic linear modes, output impedance vs Low Pass Filter input impedance? = performance degradation)
Intermodulation is very likely and unless you have the equipment to test it this that sort of concern is hard to identify / address

Allison

Thanks for your response.

I see you/Hans included negative feedback on the QRP Labs 10W amplifier,? but WA3TFS does not.? I also have a MX-P50? that has negative feedback. So I may have to find out if its needed here by test.

I agree may need to run at higher voltage, I was just looking at the Power vs voltage curves in the? 50W Amp manual, totally forgetting the the power loss on AB mode.

Looking at the layout I don't think it is that difficult to provide separate bias circuits.

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I have not been able to find a schematic for K5OOR/EBY.

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The pointer supplied was for that amp!

So I repeat.

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Allison
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I was thinking the the benefit of the QCX 50W over the WA3TFS was the use of PIN diode switching,? I need to double check the design.??

Fortunately I do have test capability.

FWIW: Part 2 is here:

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And that same person sells modern kits/boards of that amp, IIRC.??

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But, I believe there has been talk about experimentation with the QCX amp set up for linear use, but I don't know that anyone has actually tried. Might be interesting to learn if it would be relatively easy and/or stable, or very difficult.?

My comment is: Go for it! Can't hurt to try some simple tests! My only request: Please note the efficiency (DC Power in) of the amp in your tests! Might be good to know what you see for current draw as built Class C, vs linear, etc.? ? ? As I've been contemplating a small portable amplifier, I find VERY few designs talk about actual current draw or note it in testing/documentation. Lots of focus on supply voltage vs power out, but rarely the current in!?

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-Nate

N8BTR

The QRPLabs 10W linear amps and the 50W non linear amps seem to have very similar output circuitry. I am awaiting delivery of my QMX+ but I also have had for a long time one of the 10W linear amps (bought in anticipation of the QSX) that I want to use. My question is…if I removed the BS170 drivers in the 10W linear amp and added a smallish input matching attenuator would the gain be sufficiently reduced so it could be driven linearly from say a 1-2W drive from the QMX? I am really only interested in the SSB output at the higher frequencies i.e 10m and 6m and beyond, as I noticed reports of quite low powers reported generally from the QMX+ at these frequencies.

I was proposing feeding the output of the 10W amp back through the QMX+ LPFs as the old LPF modules were rated at 10W so I don’t see why the QMX+ should not handle the same. I was even considering changing the IRF PA devices for the higher frequency version used in the 5W amp (did this and got it working on 70MHz fine).

wadderya think?

73 Ken G4apb?

I think you should buy some (maybe a box of ? ) transistors and a bigger heatsink and some plain board and read up and start from there?

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When messing with input stages ?you’ll find the input impedance all over the place (hence the pi attenuator input)

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When messing with output stages the dissipation will increase and the impedance varies with the point in the cycle (so the LPF action is degraded) and remember the 50W amp has never been wideband?

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Remember the interesting way Hans avoided RF feedback at switching points by boosting the switching diode bias from the RF waveform - that probably needs checking ?too

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